This fixes #35

Note, that this is still far from an SH-compatible parser, but this at least fixes this particular issue.

To be an SH-compatible parser, these kinds of things would have to be fixed:

• HELLO=WORLD shoud be treated differently from HELLO= WORLD
• HELLO=${WORLD} should be treated differently from HELLO='${WORLD}'
• The parsing of a POSIX variable should take a previous POSIX variable in precedence over an environment variable.
• And so on.